Enthalpy relaxation in Cu_(46)Zr_(45)Al_7Y_2 and Zr_(55)Cu_(30)Ni_5Al_(10) bulk metallic glasses by differential scanning calorimetry (DSC)

摘要

Structural relaxation process in Cu_(46)Zr_(45)Al_7Y_2 and Zr_(55)Cu_(30)Ni_5Al_(10) bulk metallic glasses during annealing below the glass transition temperature T_g was investigated by differential scanning calorimetry (DSC). The features of enthalpy relaxation are sensitive to both annealing temperature and annealing time. For a given annealing time t_a, the results indicated that the relaxation time t_a decreases with increasing the annealing temperature T_a, in good agreement with results relative to other bulk metallic glasses. Additionally, the enthalpy relaxation behaviour of the bulk metallic glasses appears independent on the cooling rate used before the physical aging experiments, i.e. on the initial as-cast state. The recovered enthalpy evolution of the bulk metallic glasses is well described by the Kohlrausch-Williams-Watts (KWW) exponential relaxation function as ΔH(T_a) = ΔH_(eq){l - exp[-(t_a/τ)~β]l}. Kohlrausch exponent β and enthalpy relaxation time τ are sensitive to the composition of the bulk metallic glasses. Finally, the influence of different heating treatment processes on the enthalpy relaxation in the bulk metallic glasses is presented and shows that this phenomenon is mainly reversible. The structural relaxation behaviour is interpreted by free volume model and quasi-point defects model. Kinetic fragility parameters m in Cu_(46)Zr_(45)Al_7Y_2 and Zr_(55)Cu_(30)Ni_5Al_(10) bulk metallic glasses are 72 and 69, respectively, indicating therefore that these alloys are intermediate glasses. Crystallization process was also investigated by DSC experiments. According to the Kissinger model, corresponding activation energy is 3.18 eV in Cu_(46)Zr_(45)Al_7Y_2, and 3.19 eV in Zr_(55)Cu_(30)Ni_5Al_(10), respectively.